chore: simplify provider GetConnection

internal/provider/utils/port.go

@@ -1,7 +1,11 @@
 package utils
 
 import (
+	"errors"
+	"fmt"
+
 	"github.com/qdm12/gluetun/internal/configuration/settings"
+	"github.com/qdm12/gluetun/internal/constants"
 	"github.com/qdm12/gluetun/internal/constants/vpn"
 )
 
@@ -13,6 +17,7 @@ func GetPort(selection settings.ServerSelection,
 		if customPort > 0 {
 			return customPort
 		}
+		checkDefined("Wireguard", defaultWireguard)
 		return defaultWireguard
 	default: // OpenVPN
 		customPort := *selection.OpenVPN.CustomPort
@@ -20,8 +25,40 @@ func GetPort(selection settings.ServerSelection,
 			return customPort
 		}
 		if *selection.OpenVPN.TCP {
+			checkDefined("OpenVPN TCP", defaultOpenVPNTCP)
 			return defaultOpenVPNTCP
 		}
+
+		checkDefined("OpenVPN UDP", defaultOpenVPNUDP)
 		return defaultOpenVPNUDP
 	}
 }
+
+func checkDefined(portName string, port uint16) {
+	if port > 0 {
+		return
+	}
+	message := fmt.Sprintf("no default %s port is defined!", portName)
+	panic(message)
+}
+
+var ErrInvalidPort = errors.New("invalid port number")
+
+// CheckPortAllowed for custom port used for OpenVPN.
+func CheckPortAllowed(port uint16, tcp bool,
+	allowedTCP, allowedUDP []uint16) (err error) {
+	allowedPorts := allowedUDP
+	protocol := constants.UDP
+	if tcp {
+		allowedPorts = allowedTCP
+		protocol = constants.TCP
+	}
+	for _, allowedPort := range allowedPorts {
+		if port == allowedPort {
+			return nil
+		}
+	}
+
+	return fmt.Errorf("%w: %d for protocol %s",
+		ErrInvalidPort, port, protocol)
+}